"wolfjoe", just a few short comments in response to your post.
Magnetometers require the target to have iron or steel, and in most soils they'll find targets deeper than metal detectors will. Soils and rock with high iron mineral content can produce "false signals" which interfere with detection of weak signals from metal.
The biggest single advantage of PI for locating deep caches, is that PI doesn't require an induction balance searchcoil (in contrast to VLF/MF machines). This makes it easy to build large searchcoils. The coils designed specifically for cache hunting are usually a square frame on the order of 3 feet square, that the user steps through and supports with a strap or harness.
The other big advantage is that PI machines are less susceptible to drift than VLF/MF machines, so they can operate fully static without the "motion circuitry" or "autotuning" that most VLF/MF machines use to eliminate drift. The motion/autotune systems tend to tune out signals from large deep targets.
If you live in an area of northern climate, PI will probably do a decent job of cutting through the soil minerals. If you live in the Appalachian region, the Pacific northwest or in a tropical or subtropical climate, especially the "red dirt" areas, PI may give you fits with ground interference. Some models have provision for cancelling ground interference but this will reduce sensitivity compared to the depth you could get if the bad ground were not present.
Discrimination generally isn't of much use in cache hunting. Caches often have iron associated with them, and all discrimination schemes I'm aware of run some risk of knocking out a desirable target. "Target ID" does not entail this same risk, but I'm not aware of any PI's which offer this feature. The usefulness of target ID is primarily in "coinshooting", a task at which VLF/MF machines are generally superior to PI.
Hope this helps. There are people who frequent this forum who are more knowledgeable about PI than I am, who may be able to provide better information than I can.
--Dave J.
